This invention relates generally to underwater acoustic detection systems and more specifically to underwater doppler detection systems.
Numerous doppler sonars have been constructed in the past to detect swimmers, torpedos, or ships. Some of these systems use pulse while others employ CW in the transmitting beam. All of these systems, however, have attempted to identify targets by filtering out those frequencies which are characteristic of the target. Consequently, the main emphasis in these prior art systems has been to develop filters to accomplish this task. As a result, some doppler sonars have thousands of pounds of electronic equipment which are composed mainly of banks of filters and switches.
Other doppler sonar systems use frequency analyzers that weigh hundreds of pounds and require 110 volt power sources. The chief disadvantages of these systems are their excessive cost, complexity, and the need for sophisticated interpretation by a trained human operator. Furthermore, the filters required to isolate target doppler signals from the background are ineffective, complex, and costly. Low velocity targets provide doppler signals so close to the fundamental frequency that they cannot be differentiated from the fundamental frequency, certainly not in a portable instrument.
The frequency analyzers which have been used to replace complex filtering systems are also heavy, expensive, and difficult to handle under adverse sea conditions. Even with modern frequency analyzers the changing environmental conditions require the constant attention of a trained technician to monitor the apparatus. Finally, an automatic alarm cannot be associated with these prior art doppler sonar systems because of the low signal-to-noise ratio.